BRUSSELS—European Union foreign ministers will gather here in an emergency meeting on Thursday to discuss the EU’s response to an assault by Russian-backed rebels on the southeastern Ukrainian city of Mariupol, EU Foreign Affairs Chief Federica Mogherini said on Twitter on Sunday.

The ministers will likely discuss whether to tighten sanctions against Russia for the attack, which killed dozens of civilians on Saturday in the strategically important port city on the Sea of Azov, officials said. German Foreign Minister...